U.S. patent number 4,598,592 [Application Number 06/546,970] was granted by the patent office on 1986-07-08 for apparatus for determining the fatigue condition of a structure.
This patent grant is currently assigned to JACA Corporation. Invention is credited to Robert C. McMaster.
United States Patent |
4,598,592 |
McMaster |
July 8, 1986 |
Apparatus for determining the fatigue condition of a structure
Abstract
Electrical signals, representative of stress induced in a
structure, are developed by a piezoelectric transducer (10)
attached to the structure in response to stress changes in the
structure caused by loads applied to the structure. The signals
storage signals from the electrical signals and the storage are
processed by a signal processing circuit (12) to develop signals
are accumulated by a storage unit (14) to develop a fatigue
condition indication of the cumulative effect of the loads applied
to the structure.
Inventors: |
McMaster; Robert C. (Delaware,
OH) |
Assignee: |
JACA Corporation (Fort
Washington, PA)
|
Family
ID: |
24182784 |
Appl.
No.: |
06/546,970 |
Filed: |
October 31, 1983 |
Current U.S.
Class: |
73/786; 310/328;
310/338; 310/348; 73/787 |
Current CPC
Class: |
G01L
1/16 (20130101) |
Current International
Class: |
G01L
1/16 (20060101); G01M 005/00 () |
Field of
Search: |
;73/786,787,799,801,587,761 ;310/348,354,369 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Ciarlante; Anthony V.
Attorney, Agent or Firm: Weiser & Stapler
Claims
I claim:
1. Apparatus for determining the stress change history of a
structure, said apparatus comprising:
piezoelectric transducer means including a shaft in load bearing
association with said structure, and a piezoelectric element
surrounding said shaft and retained in compression between opposing
members so that said piezoelectric transducer means is responsive
to stress changes in said structure caused by the application of
loads to said structure for developing electrical signals
representative of stress changes applied to said structure;
circuit means responsive to said electrical signals for processing
said electrical signals to develop storage signals from said
electrical signals; and
storage means responsive to said storage signals for storing said
storage signals to develop a stress change history indication of
the cumulative effect on said structure of loads applied to said
structure.
2. Apparatus according to claim 1 wherein the piezoelectric
component of said transducer means is compressively pre-stressed in
the direction along which tension and compression forces are
applied.
3. Apparatus according to claim 2 wherein the piezoelectric
component of said transducer means is compressively pre-stressed in
three mutually perpendicular directions.
4. Apparatus according to claim 3 wherein said storage means
include an integrating unit which accumulates said storage
signals.
5. Apparatus according to claim 4 wherein said circuit means
include a full-wave rectifier for developing storage signals of the
same polarity from electrical signals representative of both
tensile and compressive loads.
6. Apparatus according to claim 5 wherein said circuit means
include level sensing means for developing storage signals only
from electrical signals which exceed a predetermined level
representative of the endurance limit of said structure.
7. Apparatus according to claim 3 wherein said circuit means are
powered by said electrical signals.
8. Apparatus according to claim 1 wherein said storage means
include an integrating unit which accumulates said storage
signals.
9. Apparatus according to claim 8 wherein said circuit means
include a full-wave rectifier for developing storage signals of the
same polarity from electrical signals representative of both
tensile and compressive loads.
10. Apparatus according to claim 1 further including a transmitter
means responsive to said storage means for transmitting said stress
change history indication to a remote location.
11. Apparatus according to claim 1 further including a transmitter
means responsive to said circuit means for transmitting said
storage signals to a remote location and said storage means are
located at said remote location.
12. Apparatus according to claim 1 further including transmitter
means responsive to said piezoelectric transducer means for
transmitting said electrical signals to a remote location and said
circuit means and said storage means are located at said remote
location.
13. Apparatus according to claim: 1 wherein said piezoelectric
transducer means is an integral element of said structure.
14. Apparatus according to claim 1 wherein said circuit means and
said storage means are remote from said transducer means and the
structure to which it is attached.
15. Apparatus according to claim 14 wherein said transducer means
is operable without an external power supply.
16. Apparatus according to claim 15 wherein said transducer means
is capable of transmitting said stress change history indication to
said circuit means and said storage means, without connecting
leads.
17. Apparatus according to claim 1 wherein said transducer means
further comprising an electrode in electrical contact with said
piezoelectric element, and a pair of nuts engaging said shaft and
retaining said piezoelectric element therebetween.
18. Apparatus according to claim 17 wherein said piezoelectric
element is compressively pre-stressed by said retaining nuts.
19. Apparatus according to claim 17 wherein a pair of piezoelectric
elements surround said shaft in annular spaced relation to one
another.
20. Apparatus according to claim 17 wherein a pair of piezoelectric
elements are separated by said electrode and retained between said
retaining nuts.
21. Apparatus according to claim 17 wherein said piezoelectric
element is surrounded by compressible members.
22. Apparatus according to claim 21 wherein said compressible
members are surrounded by resilient members.
23. Apparatus according to claim 1 wherein said storage means is a
coulombmeter.
24. Apparatus according to claim 23 wherein said coulombmeter
comprises a pair of electrodes separated by an insulating material
such that a signal applied between said electrodes causes a
conducting layer to form on said insulating material, whereby
resistance measurements between said electrodes vary in accordance
with said signal.
25. Apparatus according to claim 24 wherein said electrodes are
silver and said conducting layer is a result of silver migration
responsive to said signal.
26. Apparatus according to claim 24 wherein rectifier means
operatively couple said coulombmeter to said transducer means.
27. Apparatus according to claim 1 wherein said transducer means,
said circuit means and said storage means are operable without an
external power supply.
28. Apparatus for determining the stress change history of a
structure, said apparatus comprising:
a piezoelectric transducer adapted for attachment to a structure,
responsive to stress changes in said structure caused by the
application of loads to said structure for developing electrical
signals representative of stress changes applied to said structure,
and comprising a shaft adapted for attachment to said structure, a
piezoelectric element surrounding and spaced from said shaft, an
electrode in electrical contact with said piezoelectric element,
and a pair of nuts engaging said shaft and retaining said
piezoelectric element therebetween;
cirucuit means responsive to said electrical signals for processing
said electrical signals to develop storage signals from said
electrical signals; and
storage means responsive to said storage signals for storing said
storage signals to develop a stress change history indication of
the cumulative effect on said structure of loads applied to said
structure.
29. Apparatus according to claim 28 wherein said storage means
comprises a pair of electrodes separated by an insulating material
such that a signal applied between said electrodes causes a
conducting layer to form on said insulating meaterial, whereby
resistance measurements between said electrodes vary in accordance
with said signal.
30. Apparatus according to claim 28 wherein said apparatus is
operable without an external power supply.
31. A piezoelectric transducer for attachment to a structure and
responsive to stress changes in said structure caused by the
application of loads to said structure for developing electrical
signals representative of stress changes applied to said structure,
comprising:
a shaft for attachment to said structure;
a piezoelectric element surrounding and spaced from said shaft;
an electrode in electrical contact with said piezoelectric element;
and
a pair of nuts engaging said shaft and retaining said piezoeletric
element in compression therebetween.
Description
TECHNICAL FIELD
The present invention relates, in general, to fatigue testing of
structural parts and, in particular, to determining the stress
change history of structural parts by accumulating data
representative of the stresses to which such parts are exposed
while in service. Although the invention will be described in
connection with its use in monitoring and recording the stress
change history of a bridge, it will be apparent that it has broader
application and can be used in monitoring other dynamically loaded
structures, such as oil drilling rigs and ocean platforms, radio
and television transmitters, vehicles such as trucks, railroad
cars, earth moving machines and cranes, ship structures, building
structures, machines, hoists, cables, and the like.
BACKGROUND ART
The inspection of bridges for tne purpose of determining physical
condition and subsequent corrective action to eliminate unsafe
situations is a large and complex task. In the United States, there
are over a half million highway bridges requiring large numbers of
personnel to gather and evaluate data concerning various factors
influencing the physical integrity of these bridges. One of the
more important factors is the fatigue condition of the structural
members of the bridge and the components which fasten together the
structural members. As the bridge is exposed time varying loads
caused by the passage of vehicles over the bridge, the structural
members and fastener components may crack or rupture or be
subjected to fatigue damage by the alternating application and
relaxation of stresses caused by these loads. The net effect on
these parts is dependent upon the magnitude and frequency of the
applied loads.
Various solutions to the problems introduced by fatigue have been
put into use in the past. One approach to the fatigue problem has
been to routinely replace critical parts without regard as to
whether or not these parts actually have been damaged. This is
quite wasteful and expensive. Moreover, if the replacement is
limited to selected critical parts, the fatigue condition of other
parts is ignored.
Another approach has employed non-destructive testing methods to
determine the condition of bridge parts. Specialized
instrumentation, based for example on materials evaluation by means
of x-rays, ultrasonics, stereophotogrammetry or other tests, can be
used to examine structural members and fastener components on a
scheduled basis to determine if a part has been damaged and
requires replacement. One shortcoming of this approach is the
possibility of damage occurring resulting in failures between
examinations. In addition, test instruments which have been used to
inspect bridges are fairly sophisticated, relatively expensive and
require operation by skilled personnel.
Strain gages have been used to monitor the stresses to which bridge
parts are exposed. They are externally applied and have limited
life. Data developed over this limited time is assumed
representative of that over an extended period of time. Sometimes
certain conclusions about the past are drawn or predictions about
the future are made from these inadequate data. The risks of this
technique are apparent. There is no assurance that the loads to
which the bridge is subjected during the limited periods of
monitoring accurately represent the bridge loading over longer
periods of service.
As a practical matter, the use of strain gages to monitor the
stresses induced in bridge parts is usually restricted to limited
periods of time. Strain gages require external sources of power and
usually the strain gage unit is powered by a battery. Because
batteries become discharged or wear out, they must be replaced at
regular intervals. In addition, special care is required to protect
the battery against destruction or premature failure of the battery
which can result from exposure to the environment. This adds cost
to such units. Furthermore, strain gages are typically mounted on
exposed surfaces of members where they are subject to vandalism or
may be damaged by storms or vehicle accidents.
Still another technique employed in the past to monitor stresses to
which bridge parts are exposed has made use of a piezoelectric unit
as an acoustic emission detector and measuring device. When a part
is subjected to a load, accoustic emission occurs. By analyzing the
emission, information about the load may be developed. A serious
problem with this technique is that arrays of acoustic transducers
with sophisticated instrumentation and extensive wiring are
required to determine precisely the source and location of the
emission being analyzed. As a result, arrangements of this type are
not particularly suitable for monitoring bridge members and similar
parts. Also, if surface mounted, they are susceptible to storms,
accidents and vandalism.
DISCLOSURE OF THE INVENTION
It is an object of the present invention to provide new and
improved apparatus for determining the dynamic stress change
history of a structure.
It is another object of the present invention to provide such
apparatus which is reliable in operation.
It is yet another object of the present invention to provide such
apparatus which is self-powered and thereby avoids the need of an
external power source.
It is still another object of the present invention to provide
apparatus which provides an indication of the cumulative effect of
stresses to which a structural member has been subjected.
It is a further object of the present invention to provide
apparatus which provides an indication of the failure of a
load-bearing part.
It is still a further object of the present invention to provide
such apparatus which is relatively simple in contstruction and
inexpensive to fabricate and operate.
Apparatus for determining the stress change history of a structure,
constructed in accordance with the present invention, includes a
piezoelectric transducer adapted for attachment to a structure and
responsive to stress changes in the structure caused by the
application of loads to the structure for developing electrical
signals representative of stress changes applied to the structure.
Also included are circuit means responsive to the electrical
signals for processing these signals to develop storage signals
from the electrical signals. The apparatus further includes storage
means responsive to the storage signals for storing the storage
signals to develop a stress change history indication of the
cumulative effect on the structure of loads applied to the
structure.
BRIEF DESCRIPTION OF DRAWINGS
Referring to the drawings:
FIG. 1 is a block diagram of apparatus for determining the fatigue
condition of a structure constructed in accordance with the present
invention;
FIG. 2 is a front view, partly in section, of a piezoelectric
transducer which may be used in the FIG. 1 apparatus;
FIG. 3 is a vertical section taken along line 3--3 of FIG. 2;
FIG. 4 is a waveform diagram representative of the stresses induced
in a structure when a load is applied to the structure;
FIG. 5 is a schematic diagram of a storage unit which may be used
in the FIG. 1 apparatus; and
FIG. 6 is a block diagram of an embodiment of a signal processing
circuit which may be used in the FIG. 1 apparatus.
BEST MODE OF CARRYING OUT THE INVENTION
Referring to FIG. 1, the apparatus of the present invention
includes a piezoelectric transducer 10 adapted for attachment to a
structure and responsive to stress changes in the structure caused
by the application of loads to the structure for developing
electrical signals representative of stress changes applied to the
structure. A signal processing circuit 12, responsive to the
electrical signals developed by piezoelectric transducer 10,
processes these signals to develop storage signals. The storage
signals are stored by a storage unit 14 which develops a stress
change history indication of the cumulative effect on the structure
to which piezoelectric transducer 10 is attached of loads applied
to the structure.
The stress change history indicated by storage unit 14 can be used
a number of ways. For example, the cumulative effect on the
structure of the stresses may be compared against the fatigue life
for which the structure has been designed to determine the
remaining life of the structure. Also, the stress change history
for a prescribed period may be compared against the stress change
history for a comparable period to identify failures in the
structure.
Referring to FIGS. 2 and 3, the piezoelectric transducer may
include two piezoelectric elements 16, each in the form of a ring,
positioned at opposite faces of a copper electrode 18 which is in
the form of a disc. Each such crystal is surrounded by a silicone
rubber ring 20 which, in turn, is surrounded by a melamine
constraint ring 22. Within each crystal are a silicone rubber
packing ring 24 and a melamine ring 26. The foregoing components
are mounted on a central steel shaft 28 and held in place by a pair
of nuts 30 which engage mating threads on the shaft.
As an external load is applied to the transducer and the stresses
applied to crystals 16 change in response to this load, an
electrical signal is developed at copper electrode 18 which is
representative of the applied load. For example, the external load
may be an axial tension or compression load applied between the
outside faces of nuts 30. Shaft 28 and nuts 30 serve as electrical
ground.
Crystals 16 are able to withstand very large threedimensional
compressive stresses but are not able to withstand shear, tension
or impact forces, such as those encountered in bridge structures.
If left unprotected from such shear, tension or impact forces, the
crystals might shatter. In order to prevent this damage from
occuring, the piezoelectric transducer is preloaded to pre-stress
each of the crystals in three mutually perpendicular directions by
the application of compressive forces to the crystals. This may be
accomplished, for the embodiment illustrated, by turning nuts 30 to
axially compress cyrstals 16. As such a preload is applied equally
and from opposite directions along the axis of shaft 28, each
crystal is pre-stressed along its thickness direction along which
tension and compression forces are applied. In addition, by forming
rubber rings 20 and 24 to have a thickness greater than the
crystals they surround, the rubber rings are compressed axially as
the axial load is applied and rings 20 distort radially inward
against the crystals while rings 24 distort radially outward
against the crystals. By applying radially inward and radially
outward compressive forces against the crystals, the crystals are
pre-stressed radially. The degree of pre-stress of the crystals is
selected to exceed the anticipated maximum tensile stress and to
assure that whatever the magnitude of the tension, shear or impact
forces caused by a dynamic load, the crystal will remain in a state
of three dimensional compression and not shatter.
FIG. 4 is a waveform diagram representative of the stress
conditions of a structure to which an external load has been
applied. As an external load is applied and removed, for example
the passage of a vehicle over a bridge, the stress in the structure
fluctuates.
A piezoelectric transducer, subjected to the same stress
conditions, develops an electrical signal which is the first
derivative of the waveform of FIG. 4. In other words, the signal
from the piezoelectric transducer represents the rate of change of
the stress developed in the part being monitored.
Signal processing circuit 12 of FIG. 1 may take various forms
depending upon the nature of storage unit 14, the types of
information to be derived from the electrical signals developed by
piezoelectric transducer 10, and the ways in which such information
is to be used. In its simplest form, signal processing cirucit 12
may include an attenuator circuit and components for coupling the
electrical signals from piezoelectric transducer 10 to storage unit
14. The attenuator circuit serves to reduce the large voltage
signals developed by the piezoelectric transducer to a level for
use with the storage unit.
Likewise, storage unit 14 also may take various forms. One type of
storage unit which is particularly suited for use in the present
invention is an integrating unit, such as a coulombmeter.
Typically, a coulombmeter in accordance with the present invention
generally includes a pair of electrodes, one, for example, being
silver, which are separated by a medium which permits silver to
migrate from the silver electrode to the other electrode in
response to the application of an electrical signal between the two
electrodes. The amount of silver deposited on the second electrode
is dependent upon the magnitude and duration of the applied
electrical signal. The total build-up of silver, over a period of
time, represents the accumulation of electrical signals applied to
the device over this period of time and may be determined by
electrical resistance measurements.
FIG. 5 shows schematically a preferred form of a coulombmeter which
may be used in the present invention. This unit includes a pair of
electrodes 32 and 34 separated by a short length of insulating
material, such as glass 36. As a signal is applied between
electrodes 32 and 34, silver particles migrate from silver
electrode 32 toward the other electrode 34 along the surface of
glass 36 and become deposited on the glass surface to form a
conducting layer. As this happens, resistance measurements taken at
terminals 38 will change accordingly. Other materials besides
silver and glass may be used for the electrode and the insulating
material.
Use of a storage unit, such as the coulombmeter shown in FIG. 5,
requires the introduction of a rectifier unit in signal processing
circuit 12. The polarity of the electrical signals developed by
piezoelectric transducer 10 is dependent upon the changes of the
external load applied to the structure. Loads causing compression
of the member being monitored generate signals of one polarity,
while loads causing tension generate signals of an opposite
polarity. Whatever the direction of the load, the structure is
stressed and the fact that it is stressed is of interest. In the
absence of a rectifier, opposite polarity signals, when applied to
a coulombmeter, would remove silver from the second electrode and
cause the silver to return to the silver electrode. Besides not
recording such stresses of the structure, indications of stresses
caused by oppositely directed loads are cancelled. By including a
full-wave rectifier in signal processing circuit 12, all the
storage signals developed by the signal processing circuit have the
same polarity, regardless of the nature of the external load and
the indication developed by storage unit 14 represents stresses
induced in the structure, caused by both tensile and compressive
loads.
FIG. 6 is a block diagram of one embodiment of a signal processing
circuit which may be used in the present invention. Electrical load
signals from the piezoelectric transducer are simultaneously
supplied to an attenuator 40, a timer 42, and a power supply 44.
Attenuator 40 serves to reduce the electrical load signals to a
level at which they eventually may be supplied to the storage unit
after being processed.
The attenuated electrical load signals are supplied to a rectifier
46 which produces storage signals of the same polarity. These
storage signals may be supplied directly to the storage unit or, as
shown, they may first pass through a peak detector 48 and a level
detector 50. The peak detector may be provided if information
concerning the magnitude and frequency of specific loads is
desired.
Level detector 50 is particularly useful when the structure being
monitored is fabricated from a material having an endurance limit.
The endurance limit of a material is that stress level below which
an applied load has virtually no effect on the fatigue life of a
part made from that material. The dashed line in the waveform of
FIG. 4 represents the endurance limit. In order to discriminate
between electrical signals from piezoelectric transducer 10
developed from stresses above and below the endurance limit of the
structure being monitored, level detector 50 may be provided. This
unit functions to pass to storage unit 14 only those signals
developed in response to stresses which are in excess of the
endurance limit. As a result, storage unit 14 develops an
indication of the fatigue condition of the structure from only
those signals which affect the fatigue life of the structure. For
structures made from materials not having an endurance limit, level
detector 50 would be eliminated from signal processing circuit 12.
Attenuator 40, rectifier 46, peak detector 48 and level detector 50
all may be of conventional construction and operation.
Timer 42, also of conventional construction and operation, serves
to develop timing signals to control the operation of rectifier 46,
peak detector 48 and level detector 50, if other information
besides cumulative stress is desired. For example, indications of
the durations of the loads may be developed. When applied to a
bridge construction, this information may be used to detemine the
speed of a vehicle crossing the bridge.
Power supply 44 serves to develop, from the electrical signals,
power required to operate the components of the signal processing
circuit. The magnitude of the electrical power output from
piezoelectric transducer 10 is dependent upon the number and size
of the piezoelectric elements 14. The transducer can be arranged to
provide sufficient power to operate the circuitry for processing
the information signals as well as transmitting useful information
representative of the stresses to which the structure has been
subjected. In this way, external power is not required. Instead,
the forces which subject the structure to stresses and about which
information is developed are the original source of power. Power
supply 44 also may be of conventional construction and
operation.
Piezoelectric transducers of the type which are usable in the
present invention can be attached to the structure being monitored
either as a component which is specially adapted to form part of
the structure being monitored or they can be secured to the
structure being monitored to function strictly as a sensing member.
For example, shaft 28 in FIG. 2 may be the shank portion of a
structural bolt and crystals 16 and the associated parts may be
arranged as a washer unit under the head of the bolt or under a
mating nut. Alternatively, the piezoelectric transducer can be
arranged to be secured externally to a structural part which is to
be monitored.
FIG. 1 shows a transmitter 52 in dashed lines. This is intended to
indicate that the apparatus of the invention may be arranged to
transmit the various types of information to a remote location
instead of requiring that a reading be taken at the location of the
apparatus. The electrical signals developed by piezoelectric
transducer 10 are large enough to power a transmitter, so that an
external source of power is not required. In this way, the
continuously developed indication of fatigue condition may be
received at a location remote from the site of the structure and
hazardous conditions can be discovered immediately.
Alternatively, the output storage signals from signal processing
circuit 12 may be transmitted to a remote location at which the
storage unit is located and the stress change history indication is
developed at this remote location. Also, both the signal processing
circuit and the storage unit may be located at a remote location,
in which case only the piezoelectric transducer is positioned on
the structure being monitored. For this arrangement, the electrical
signal output from the piezoelectric transducer is transmitted to a
remote location by means of an antenna 54.
While in the foregoing there have been described preferred
embodiments of the present invention, it should be understood by
those skilled in the art that various modifications and changes can
be made without departing from the true spirit and scope of the
invention as recited in the claims.
* * * * *